A thermal airflow meter, which measures an intake air flow rate to an engine, has a problem in which characteristics of the thermal airflow meter is deteriorated due to stain, such as dust, contamination, oil deposits, and the like. The thermal airflow meter will be referred to as AFM, hereinafter.
JP-4-174325A shows an AFM provided with a stain remover which removes a stain every when a vehicle travels a specified distance. However, in this AFM, deterioration in characteristics of the AFM is not diagnosed. Thus, even though the AFM is stained and its characteristics are deteriorated, it is likely that the stain is not removed. That is, it is desirable that a degree of deterioration due to stain should be determined.
Japanese Patent No. 3075877 shows a malfunction determination apparatus for an AFM. This malfunction determination apparatus has a determination means for determining whether an AFM has a malfunction according to an output level therefrom, a voltage variation obtaining function which obtains a variation in voltage applied to the AFM from a battery. After a voltage variation is obtained, the determination means starts to determine whether the AFM has a malfunction when a predetermined condition is established. In this malfunction apparatus, it is likely that the apparatus erroneously determines that the AFM has a malfunction, due to a variation in air flow rate.
JP-2000-97101A shows that a measurement error of an AFM is corrected in view of a variation in air flow rate due to pulsations when it is determined whether the AFM has a malfunction based on the measurement error. The variation in air flow rate is caused by a variation in engine speed, engine torque and driving conditions of the other relating components, such as an EGR valve, a throttle valve, a turbocharger and a variable valve timing mechanism. However, in JP-2000-97101A, such a driving condition of the other component is not considered. In order to correctly obtain the measurement error of the AFM, it is necessary to make the air flow rate more stable.